The connection of metallic wing elements to the metallic central box of the fuselage of an aircraft traditionally calls for cross-shaped or T-shaped junction elements provided with longitudinal branches, bolted onto the wing and the central box panels opposite each other, provided with a lower branch, onto which is bolted a rib, and, in the case of cross-shaped elements, with an upper branch onto which is bolted a higher fuselage panel.
The lower branch receives a rib from the central box upon which longitudinal compression fittings are supported opposite each other on the rib, each one of the fittings being bolted underneath a stiffener.
According to this design, the upper wing panels and central box panels are sandwiched between the longitudinal branches of the cross-shaped or T-shaped junction elements and the stiffeners, the latter being themselves fastened together through compression fittings. The compression stress that the wings exercise upon the central box being for the most part taken up by the cross-shaped or T-shaped element, the compression fittings, on account of their being distant from the panels, participate little in the taking up of the stress.
For this type of embodiment, the division of the stress is on the order of 75% in the cross type junction element and 25% in the fittings, which requires sizing the cross-shaped or T-shaped junction element in a consistent fashion.
In the case of an aircraft comprising traditional wings with metallic panels and stiffeners and a central box made of composite materials, the connection of the wings to the box is complex because the composite boxes are constructed by using panels with integrated composite stiffeners with a low stiffening rate and without a heel, and which therefore cannot be used as fastening points of compression fittings.
In addition, for aircraft with a large wingspan, the sizing of the junction element produced by die-forging becomes problematic, its weight becomes too considerable, the technology used reaches its limits in terms of acceptable flux level, and its manufacture becomes difficult.